Generally, a filter refers to a system that performs a specific operation in response to an input signal and generates an output signal based on the operation result. More specifically, the filter may refer to a circuit designed to remove an undesired portion of a frequency spectrum so as to obtain a desired transmission characteristic.
Typical filters, which are widely used in the field of communications, include low-pass filters (LPFs) that allow only low-frequency signals to pass therethrough, high-pass filters (HPFs) that allow only high-frequency signals to pass therethrough, and bandstop filters (BSFs) that cut off signals in a specific frequency band.
Such a filter is manufactured by appropriately combining passive devices, such as a resistor (R), an inductor (L), and a capacitor (C), and frequency characteristics of the filter are dependent on the circuital arrangement and device characteristics of the combined passive devices.
In recent years, communication systems using increasingly higher frequencies (e.g., microwaves and millimeter waves) have been introduced in order to utilize conventional deficient communication channels more efficiently. In particular, when a communication system uses an ultrahigh frequency, such as microwaves and millimeter waves, the communication system can be scaled down. Therefore, owing to the increased demand for miniaturization, communication systems using ultrahigh frequency bands have become strongly relied upon.
However, a conventional filter in which passive devices, such as a resistor (R), an inductor (L), and a capacitor (C), are mounted on a printed circuit board (PCB) may not be applied to ultra-high frequency communication systems. This is due to the fact that a high frequency leads to a short wavelength which thereby worsens interference between communication lines so that each of the communication lines operates as a circuit device. In other words, since unpredictable elements are increased in ultra-high frequency bands, there is a specific technical limit for employing typical passive devices in the ultra-high frequency bands.
Therefore, a vast amount of research has been conducted on developing passive devices applicable in ultra-high frequency bands, such as microwaves and millimeter waves. For example, conventional methods have been used in an attempt to embody two-dimensional lumped elements so as to predict parasitic elements in high-frequency bands.
However, conventional ultra-high frequency pass filters cause high signal loss in the pass band and frequently allow frequencies other than target frequencies, particularly, spurious harmonic frequencies, to pass therethrough. To address the above-described problems and improve the characteristics of filters, it may be desirable to reduce characteristic impedances. However, since the size of a filter must be increased to reduce the characteristic impedance, it is difficult to embody filters that are usable in ultra-high frequency bands and have good frequency characteristics.